1. Field of the Invention
This invention relates to an optical scanning apparatus, and more particularly to an optical scanning apparatus to be incorporated in an image forming apparatus.
2. Description of the Related Art
In general, in an image forming apparatus such as a laser beam printer, scanning of laser beams is realized by causing a rotating polygon mirror to reflect the laser beams irradiated from a laser diode.
Recently, high-speed print and high image quality are strongly demanded. In order to realize this demand, a multi-beam method capable of printing out multiple lines by one scanning with the use of multiple beams is increasingly becoming mainstream.
In the multi-beam method, the main scanning position of a beam spot on a photosensitive member may be misaligned from an ideal position due to various reasons.
According to Japanese Patent Laid-Open No. 09-96771, a method is proposed in which an image writing position is determined by detecting synchronization signals corresponding to respective beams with one photo detector. A technique obtained by further improving this technique is also proposed (Japanese Patent Laid-Open No. 2001-228417). According to Japanese Patent Laid-Open No. 2001-228417, a method is proposed in which a writing position is determined based on position data corresponding to misalignment from a virtual straight line. This virtual straight line is specified by a cutout formed in the stem of a laser diode.